The cracking furnace forms the heart of many chemical manufacturing processes. Often, the performance of the cracking furnace largely determines profitability and it is therefore extremely desirable to maximize such performance.
In manufacturing processes such as the production of ethylene, feed hydrocarbon gas such as ethane, propane, butane and/or naphtha, gas oil is fed into the cracking furnace. A diluent such as steam is usually combined with the feed hydrocarbon forming a gaseous feed or gaseous process stream which is converted in the furnace, to a gaseous mixture, which primarily contains hydrogen, methane, ethylene, propylene, butadiene and other by-products. At the furnace exit, this mixture is cooled, enabling removal of most of the heavier products, and compressed.
The compressed mixture is routed through various distillation columns where the individual components, mostly ethylene and propylene, are purified and separated and removed from the ethylene plant to be used in variety of secondary products.
The primary function of the cracking furnace is to convert the feed stream to ethylene and/or propylene. As well known, a semi-pure carbon, which is termed "coke", is formed on the metal surfaces of the cracking tubes/coils which are in contact with the feed stream and on the metal surfaces of the heat exchangers which are in contact with the gaseous effluent and product mixture from the cracking furnace, as a result of the furnace cracking operation. Coke formation generally results from a combination of a homogeneous thermal reaction in the gas phase and a heterogeneous catalytic reaction between the hydrocarbon in the gas phase and the metal in the walls of the cracking tubes or heat exchangers (catalytic coking).
In addition to the above, well recognized, locations of coke deposit, coke might form on connecting conduits and other metal surfaces, which are exposed to hydrocarbons at the high temperatures. A more subtle effect of coke formation occurs when coke enters the furnace tube alloy in the form of a solid solution. The carbon then reacts with the chromium in the alloy and chromium carbide precipitates. This phenomenon, known as carburization, causes the alloy to lose its original oxidation resistance, thereby becoming susceptible to chemical attack. The mechanical properties of the tube are also adversely affected. Carburization may also occur in respect of iron and nickel in the alloys.
A number of heat-resistant alloys have been proposed for improving high-temperature strength, corrosion-resistance and heat-resistance. However, as no such alloys suppress coke deposition by promoting a water-gas reaction in the cracking furnace, conventionally, a large amount of steam has been added to the hydrocarbons being cracked, thereby suppressing coke deposition to a certain degree. However, the degree of suppression is still not sufficient to prevent coke deposition.
A common operating procedure for a cracking furnace is to periodically shut down the furnace in order to burn out the deposits of coke. This downtime results in a substantial loss of production. In addition, coke is an excellent thermal insulator. Thus, as coke is deposited, higher furnace temperatures are required to maintain the gas temperature in the cracking zone at a desired level. Such higher temperatures increase fuel consumption and will eventually result in shorter tube life.
The period during which thermal cracking of hydrocarbons can be continued without decoking is usually from 10 to 60 days, depending on the hydrocarbon feed quality and the severity of the pyrolysis conditions. Approximately, one to three days are required for decoking, resulting in a decrease in the amount of ethylene, propylene etc. produced. In addition, when decoking not only must an enormous amount of steam be used but an amount of fuel sufficient to heat the thermal cracking coil is required.
There have been numerous attempts over very many years to suppress coke deposition, thereby making long-term, continuous thermal cracking of hydrocarbons possible, for example, by incorporating sulfide, sulfate or thiosulfate of alkali metals or alkaline earth metals into the hydrocarbon feed, as inhibitors; precoating or plating the tubes with noble or other metal alloy; applying a protective film of halogen containing silanes, disilanes and siloxanes, and converting the silicon compound to silicon dioxide; and applying a protective film of a metal-ceramic material containing aluminum oxide dispersed in chromium. on the inner walls of furnace tubes.
U.S. Pat. Nos. 2,893,941; 3,617,478; 4,542,253; 4,410,414; 4,889,614; 5,358,626; 5,435,904; 5,565,087 and UK 1,578,896 are examples of some of many prior proposals utilising antifoulant solution, by injection, in the main, into the cracking coils and, in so far as they may be relevent, the disclosures of the above patents are incorporated herein by reference.
In particular, U.S. Pat. No 5,435,904 teaches the injection of a liquid tin-containing antifoulant into a saturated hydrocarbon feed stream for a thermal cracking reactor so as to alleviate the undesirable formation of coke and carbon monoxide during subsequent thermal cracking of light hydrocarbons. However, although the time of the run is increased, coke deposits still accumulate on the inner walls of the cracking tubes requiring periodic shutdown to remove the coke deposits.
None of the approaches taught by the prior art which teach injection of antifoulant solutions to suppress the catalytically carbonizing action of nickel and iron of heat-resistant alloys, has been entirely satisfactory particularly as the prior apparatus for injecting the solution into the gaseous stream have not been effective in providing cover of the antifoulant over the entire inner surfaces of the furnace tube.
Accordingly, a process which could effectively decoke coils or tubes within a thermal cracking furnace without having to raise the wall temperatures within coils to perform a decoking process and without having to shut down the furnace, would be highly desirable.
As generally known, a solution may be dispersed into a process gas stream in several conditions. However, a disadvantage arises when the solution is injected as a continuous stream as the solution may reside on a support surface as a statistical film, only one side of which contacts the process gas stream. Subsequent evaporation of the liquid, leaves solid residues (additives or solute) deposited on the support surface so that a substantial part of these additives is inactive. Furthermore, the deposition of the solution and subsequent evaporation may result in damage to the furnace tube.
However, dispersal of a solution into small drops, which can be dynamically supported in any suspended conditions enables soluble additives to be distributed into any coil such they will be covering all inner surface of a coil. When liquid is disintegrated to drops the interphase surface is increased. For so example, the disintegration of a drop with a diameter of about 1.0 millimeter into droplets with diameters of about 1 micron increases the interphase surface one thousandfold, from 0.0314 to 31.4 square centimeters.
Drops can disintegrate to very small sizes by interaction with a process gas stream. To obtain drop instability, for example, for a water drop with diameter about 1 mm in a process gas stream, the stream velocity should be about 15 meter/sec. To obtain a reduction of drop diameter to about 100 micron, the velocity must be increased about threefold (3.3).
Reference is made to the section on drop formation in Chemical Engineer's Handbook edited by John Perry and published by McGraw-Hill Book Company in 1963, the disclosure of which is incorporated herein by reference.
The process gas density, viscosity, and surface tension, and the density of the liquid all affect the resulting drop size. Increasing the process gas density decreased the drop stability, resulting in smaller drops. Increasing liquid viscosity and surface tension are increased increases drop size.
The process of drop decomposition causes liquid to cross from high pressure zones to low pressure zones causing the liquid to evaporate very quickly. As a result of the large interface surfaces a spray of small drops is quickly vaporized.